Spatial light modulator

ABSTRACT

A spatial light modulator formed of a moveable electrode which is disposed opposite a fixed electrode and is biased to roll in a preferred direction upon application of an electric field across the electrodes to produce a light valve or light shutter. In one embodiment, the moveable electrode is restrained at one end and coils about the fixed end in a preferential roll direction. The bias is achieved by inducing anisotropic stress or anisotropic stiffness.

This invention was made with government support under Contract NumberF19628-90-C-0002 awarded by the Air Force. The government has certainrights in the invention.

The present application is a National Phase Filing of PCT InternationalPatent Application Number PCT/US93/06342 filed on Jul. 2, 1993, whichclaims priority as a continuation-in-part from U.S. patent applicationSer. No. 07/655,345 filed Mar. 6, 1991, now U.S. Pat. No. 5,233,459issued on Aug. 3, 1993.

BACKGROUND OF THE INVENTION

This invention relates to spatial light modulators and devices madetherefrom; such as display devices and printers. Electric displaydevices convert electric signals transmitted from electric or electronicdevices into optical signals that can be recognized by humans. Theoptical signal is displayed as an image in the form of digits,characters, or graphics. Electric displays are divided into active orpassive types. When the optical signal is displayed by light emission,the display is termed an active display, whereas when the display worksby modulating incident light through reflection, scattering,interference, or similar phenomena, using an external light source it istermed a passive display.

Displays may be further subdivided into several further categories, asfollows:

LCD liquid crystal display;

ECD electrochemical display;

EPID electrophoretic image display;

SPD suspended particle display;

TBD twisting ball display;

PLZT transparent ceramics display; and

ELVD electrostatic light valve display.

An ELVD is described in U.S. Pat. No. 3,989,357 issued Nov. 2, 1976 toCharles G. Kalt. Kalt's ELVD is a passive device and consists of a fixedelectrode to which a moveable, coiled, resilient sheet electrode isattached with an insulating layer separating the two electrodes. Thecoiled electrode is caused to unroll upon application of an electricpotential between the two electrodes. The coiled electrode thus acts asa light shutter.

The inner surface of the coiled electrode has a color or reflectivitythat is different from that of the fixed electrode, in which case, thedevice to an observer changes hue or reflectivity when a potential isapplied. Alternatively, the fixed electrode may be transparent to aselected portion of the electromagnetic spectrum and the coiledelectrode may be opaque. In this case, the device is said to operate inthe transmissive mode and a light source positioned behind the devicewould transmit light when no potential is applied and would not transmitlight when a potential is applied.

Other patents pertaining to ELVD's are U.S. Pat. Nos. 3,897,997,4,094,590, 4,235,522 and 4,248,501.

The above patents generally teach the use of metallized plastic sheetsas the moveable or coiled element. These sheets are formed into a rollusing heat and a mandrel or by bonding two plastic sheets, one of whichis prestressed in one direction before bonding. There are a number ofproblems associated with these approaches when considering manufacturingcost, reliability, temperature effects and electrical charge control inthese devices. The methods described require individual handling of eachshutter to form the roll and to bond it to the fixed electrode. Some ofthe problems of handling are described in U.S. Pat. No. 4,094,590, whichdescribes the formation of wrinkles. The ELVD's of Kalt are intended forlarge aperture devices and the process is not suitable for smallaperture devices of about 0.004" square. These prior art devices rely onthe elastic properties of plastic, which is undesirable, since theseproperties can vary widely with temperature and humidity and oftenchange as they age. The flexing characteristics of the moveableelectrode are determined by these elastic properties, therefore, thethreshold voltages are likely to drift. U.S. Pat. Nos. 4,235,522 and4,248,501 describe some of the issues of charge control in theinsulator. These problems are more severe than is indicated in thepatents. Even small amounts of accumulation or drift of charge in theplastic materials described will cause large amounts of threshold driftin the light valves, which is undesirable for many applications.

U.S. Pat. No. 4,729,636 discloses an ELVD in which an electrostaticallymoveable apertured non-rotatable electrode is disposed between two fixedelectrodes and insulated therefrom. The electrode structure is immersedin a liquid of contrasting color with respect to the surface color ofthe moveable electrode. The moveable electrode can be moved back andforth between two stable positions. To an observer, the color of thepicture element at each electrode changes from that of the moveableelectrode to that of the liquid depending upon the position of themoveable electrode.

SUMMARY OF THE INVENTION

A lower cost method for mass production of small apertured, high speedELVD's is disclosed. The process uses film deposition methods andlithography for making the valves. This means that all of the electrodescan be made from thin films. The term "thin films" is used to denote afilm formed of layers of material deposited by some type of atom by atomor molecule by molecule process, rather than layers produced by layeringdown relatively large particles, thinning material or by rolling. Thinfilms are different in their properties because they are characterizedby small crystal grain size and are sometimes even amorphous.

Thin films are different material from bulk material because of thegrain size. They generally have a larger tensile strength than bulkmaterial. Except for electroplating processes, thin film depositionrequires a vacuum or low pressure environment. In practice, thisrequires vacuum evaporation, sputtering or chemical vapor deposition(CVD), plasma deposition, molecular beam epitaxy (MBE), ion beamsputtering, or other similar process. Large numbers of very smallaperture area shutters (sometimes referred to herein as microshutters orvalves) can be made simultaneously over large areas when using thin filmand lithography techniques.

Small aperture devices have many advantages such as higher resolution,higher speed, lower voltage operation and easier fabrication. Ingeneral, therefore, the invention comprises an electrostatic light valveor shutter and a method of forming such a valve. The valve consists of afixed electrode and a rotatably moveable electrode with an insulatorbetween the two electrodes. When the moveable electrode is moved towardthe fixed electrode by application of an electrostatic force, nometal-to-metal contact occurs. In the transmissive mode, the fixedelectrode is transparent and the moveable electrode is opaque and actslike a shutter.

The insulator reduces the transfer of charge from one electrode toanother. The charge transfer reduces the holding force on the moveableelectrode and allows it to move away from the fixed electrode, therebyopening the shutter.

Preferably, the moveable electrode is an anisotropically stressed orstiffened electrode which is stressed or stiffened as formed. Thisanisotropic characteristic could be considered as a mechanical bias or amechanical polarization. The anisotropic stress or stiffness causes theelectrode to rotate in a preferential direction, i.e., where the stressis greater, or perpendicular to the direction of stiffening. Theanisotropic stiffening may be induced by forming periodic corrugationsin the electrode to stiffen the electrode in a direction orthogonal tothe preferential direction. Anisotropic stress may be induced asdeposited by forming the electrode in a deposition process whichproduces anisotropic stress.

In a first embodiment the moveable electrode is a coilable electrodefixed at one end which rolls up in a preferred direction and unrollsupon application of an electric field across the electrodes. In analternate embodiment the moveable electrode is a deformable membranefixed at both ends. In yet another embodiment, the moveable electrode ishinged.

An embodiment of the invention may comprise an array of anisotropicallystiffened electrostatically moveable electrodes separated from an arrayof fixed electrodes by one or more insulative layers. Such an array maybe made in accordance with a method of the invention, as follows:

A thin transparent conductive layer is formed on a suitable substrate,such as glass. A photoresist layer is formed over the conductive layerand patterned using conventional lithography techniques. The exposedconductive layer is then etched away, leaving individual electrode orpixel areas; using the resist as a mask. The mask is removed and thepatterned film of fixed electrodes is then covered with a thintransparent insulator film. Edges of the electrodes where contacts willbe formed are suitably masked prior to forming the insulator film.

Another layer of photoresist is formed over the patterned structure andpatterned so as to leave a series of resist regions over the individualelectrode areas. The substrate with resist is then heated for a shortperiod, after which a third resist layer is applied and patterned toleave a series of resist regions extending across the width of theelectrode regions. The structure is again heated at elevated temperaturefor a short period. The series of resist regions form anisotropicstiffening corrugations and a release layer, for an overlying coilableelectrode. The coilable electrode is formed by depositing and patterningsuccessive layers of: i) a low stress insulating film, ii) a conductivefilm having stress of one type, i.e., compressive, and iii) a conductivefilm having stress of opposite type (tensile).

An optional low stress protective coating is then formed over thisstructure and the structure is patterned to define contact areas wherecontact metallization is deposited. Photoresist and etching is then usedto further define pixel areas and to completely remove the resistrelease layer, whereupon the anisotropically formed moveable electrodescoil up in the intended roll direction. Coiling occurs because theelectrode is formed from a bottom low stress insulative layer, and acombined conductive compressive/tensile stress layer and a top lowstress protective layer. Corrugations in the electrode provide lateralstiffness to prevent the electrode from curling perpendicular to theintended roll.

Alternative fabrication processes are described. In one process, ahinged shutter is formed using a dry etch process. In yet anothermethod, a low temperature freeze drying process for forming coiledELVD's is described. The principles of the invention may also beutilized to form devices other than ELVD's. For example, as will beshown, a micro-electro-mechanical switch may be formed for switchingmicrowave power or to form a micro-mechanical relay. Anelectrophotographic printer employing a linear microshutter array of theinvention is also described herein.

The above and other features and advantages of the invention will now bedescribed in detail in connection with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(j) are a series of schematic sectional views showing thesteps in fabricating a valve for an electrostatic light valve display(ELVD) in accordance with the invention.

FIG. 2 is a schematic plan view of a 5×5 matrix array of ELVD's in whichthe pixel shutters are shown open.

FIG. 3 is a sectional view along lines III--III of

FIG. 2, having an additional cover in place.

FIG. 4 is a view as in FIG. 2 showing the pixel shutters in the closedposition.

FIGS. 5(a)-(j) are a series of schematic cross-sectional views showingsteps in fabrication of an alternate embodiment of the invention.

FIG. 6 is a schematic cross-sectional view of a transmissive mode colorELVD embodiment.

FIG. 7 is a schematic cross-sectional view of an active transmissivemode color ELVD embodiment.

FIG. 8 is a schematic cross-sectional view of an ELVD with an array oflenslets to enhance contrast.

FIG. 9 is a schematic cross-sectional view of a reflective mode ELVD.

FIG. 10 is a schematic cross-sectional view of two shutter array ELVDoperated in the reflective mode.

FIG. 11 is a timing diagram showing the applied DC voltages required foroperation of the ELVD.

FIG. 12 is a timing diagram as in FIG. 11 for AC mode of operation.

FIG. 13 is a timing diagram showing how the bistable nature of the ELVDcan be used to switch pixels by employing a combination of vertical andhorizontal applied voltages.

FIG. 14 is a schematic of a projection display using an ELVD.

FIGS. 15(a)-(c) are cross-sectional views illustrating certain chargeconcepts of the invention.

FIG. 16 is a sectional view of an ELVD with a dual radius.

FIG. 17 is a sectional view of an alternate dual radius ELVD embodiment.

FIG. 18 is a sectional view illustrating how the dual radius ELVD may beformed.

FIGS. 19(a) and 19(b) are cross-sectional views (voltage off) 19(a) and(voltage on) 19(b) of an ELVD with a rail or spacer gap construction.

FIG. 20 is a schematic plan view of an ELVD pixel with bars or railsextending along the roll direction and wherein the moveable electrode isrolled up.

FIG. 21 is a section along lines XXI--XXI of FIG. 20.

FIG. 22 is a schematic as in FIG. 20 with the electrode unrolled.

FIG. 23 is a section along lines XXIII--XXIII of FIG. 22.

FIG. 24 is a magnified cross-section of an ELVD illustrating the steppedconstruction of the moveable electrode.

FIG. 25 is the same section, as in FIG. 24, with the electrode furtherunrolled.

FIG. 26(a) is a cross-sectioned view of a multicolor display.

FIG. 26(b) is a plan view of the display of FIG. 26(a).

FIGS. 27(a) and 27(b) are cross-sectional views of a deformable membraneswitch (DMS) display embodiment operating in the reflective mode showingthe switch Off FIG. 26(a)! and On FIG. 26(b)!.

FIGS. 28(a) and 28(b) show an alternate embodiment of a DMS in the Offand On positions, respectively.

FIGS. 29(a)-29(l) are a series of schematic cross-sectional viewsshowing key steps in the fabrication of another alternate embodiment ofthe invention.

FIGS. 29(x)-29(z) and 29(z') are top plan views of the cross-sectionalviews of FIGS. 29(a)-29(l) at various steps in the process.

FIGS. 30(a)-30(j) and FIGS. 30(l)-30(o) are a series of cross-sectionalviews of the main steps in fabrication of a low cost embodiment of anELVD of the invention.

FIGS. 30(k) and 30(p) are plan views of the cross-sectional views ofFIGS. 30(j) and 30(o), respectively.

FIGS. 31(a)-31(i) are a series of schematic cross-sectional viewsshowing key steps in the fabrication of shutters for ELVD's using afreeze drying process.

FIGS. 32(a)-32(i) are a series of schematic cross-sectional viewsshowing key steps in the fabrication of a hinged shutter for an ELVD.

FIG. 33 is a top plan view of a hinged microshutter at one stage ofprocessing.

FIG. 34 is a top plan view of a hinged microshutter at a later stage ofprocessing.

FIG. 35 is a top plan view of a hinged microshutter at a last stage ofprocessing.

FIG. 36(a) and 36(b) are enlarged partial sectional views of an ELVDtaken after film deposit before release FIG. 36(a)! and after releaseFIG. 36(b)!.

FIG. 37 is a perspective view of an ELVD embodiment with anisotropicstress induced by a series of stress strips.

FIG. 38 is a schematic sectional view of a shutter in a partially rolledout state.

FIG. 39 is a partially exploded perspective view of a partially rolledout shutter.

FIG. 40 is a section taken along lines A--A of FIG. 38.

FIG. 41 is a section taken along lines B--B of FIG. 38 with thepotentials on the electrodes in a hold mode.

FIG. 42 is a section taken along line B--B of FIG. 38 with thepotentials on the electrodes in a roll-out mode.

FIG. 43 is a sectional view of an ELV with an electrically floatingshutter, shown in the open position.

FIG. 44 is a sectional view, as in FIG. 43, showing the switch in theclosed position.

FIG. 45 is a perspective view of an alternate embodiment of anelectrically floating ELV shutter.

FIG. 46 is a schematic drawing of a printer system using a shutter arrayof the invention as a print head.

FIG. 47 is a side view of the print head of FIG. 45.

FIG. 48 is a plan view of the linear microshutter taken in the directionA--A of FIG. 47.

FIG. 49 (a) is a side view of a DC switch of the invention shown in theopen position.

FIG. 49(b) is a side view of a DC switch of the invention shown in theclosed position.

FIGS. 50(a)-50(o) are sectional and plan views of an alternate DC switchembodiment illustrating the construction thereof.

FIG. 51 is a sectional view taken along lines A--A of FIG. 50(o).

FIG. 52 is a sectional view taken along lines B--B of FIG. 50(o).

FIG. 53 is a sectional view taken along lines C--C of FIG. 50(o). FIG.54 is a plan view of a 4 by 4 array of DC switches. FIG. 55(a) is aschematicized cross-sectional view of microwave switch of the inventionin the open position.

FIG. 55(b) is a view as in FIG. 55(a) showing the switch in the closedposition.

FIG. 56 is a plan view of an array of microwave switches shown in theclosed position.

FIG. 57 is a sectional view along lines A--A of FIG. 56.

FIG. 58 is a plan view of an array of microwave switches shown in theopen position.

FIG. 59 is a sectional view along lines A--A of FIG. 59.

DETAILED DESCRIPTION OF THE INVENTION Coiled ELVD Process

Referring now to FIGS. 1(a)-1(j), a preferred process for forming thedisplay apparatus of the invention will be described in connectiontherewith.

The process depends somewhat on the substrate material used. Becauseglass is inexpensive and commonly available with flat smooth surfaces,it presently is a preferred substrate and has therefore been chosen asthe substrate material for this description. Other prospectiveequivalents may comprise fused quartz, or in reflective mode operation,single crystal silicon, ceramic, metals, or the like. The first stepFIG. 1(a)!, after cleaning and inspection to determine that thesubstrate 10 has sufficiently good surface quality, is to coat thesubstrate with a thin transparent conductive layer such as 1000-3000 Åindium tin oxide (ITO). A sputtering machine with a sputtering targetcomposed of indium oxide with about 10% tin oxide may be used to formthe transparent conductor film layer 12. Note that ultra-thintransparent films of metals, such as, gold, platinum, silver or tungstenmay be used in place of the ITO film. The ITO coated glass plate is thenover coated with photoresist and the resist is patterned usingconventional lithography techniques and the exposed ITO is then etchedwith an ITO etch using the photoresist as a mask (not shown). This stepdefines the lower or fixed electrode. Next, the patterned ITO film 12 iscovered with a high resistance film 14, such as a silicon dioxide filmusing a chemical vapor deposition system. Preferably the thickness offilm 12 ranges from about 50 Å to 5000 Å but may extend up to 25,000 Åas desired. Alternatively, one could form the high resistance film ofsilicon nitride deposited in a sputtering machine using a silicon targetand nitrogen or a nitrogen argon mixture as the sputter gas. As afurther alternative, one could deposit silicon dioxide using asputtering or evaporation system.

Before depositing the silicon dioxide film 14, the edges of the platewhere the wire contacts will be made are covered with masking strips(not shown) to block deposition. The silicon dioxide film 14 provides ahigh resistivity layer which will be disposed between the fixedtransparent electrode conductor 12 and the top coilable electrode to beformed next. The structure is then coated with photoresist 16 (FIG.1(b)! having a thickness around 3000 Å using conventional techniques.The photoresist may be a negative or positive resist. The positiveresist is comprised of a base resin (Novolak) and a photoactive compoundand is patterned, again using conventional techniques, leaving resist inareas where release in an overlying film is required. A resist, such asthe photoresist sold under the name "AZ 1350B", has been used with goodresults. The structure with resist is then baked at 400° C. for oneminute in air, FIG. 1(c) transforming the resist layer into a smoothhard layer 16'. This bake process partially carbonizes the resist makingit very hard and inert. The films formed on the baked resist can havethe same properties as films on glass with a wide variety of materialsusing various deposition processes. A second layer of resist for formingcorrugations is then applied as before, and patterned in a 4 micronperiod grating of resist regions 18 over the previously baked resistareas FIG. 1(d)!. The structure is again baked at 400° C. for one minuteforming a second smooth periodic hard resist structure 18' FIG. 1(e)!.This second resist layer 18' provides the corrugations in an overlyingelectrode formed in the next steps. The two resist layers 16' and 18'together provide the release layer for the overlying electrode. Thepatterned structure is then coated with a 300 Å thick film 20 of highresistivity material, such as silicon dioxide or other alternativematerial, as before. The stress of this film 20 FIG. 1(f) must be lowcompared with the overlying layers or have compressive stress. Since theamount of stress will affect the tightness of the coil of themicroshutter, the level of stress must be carefully controlled. Stresscontrol is achieved by carefully selecting appropriate depositionprocesses and conditions. For example, silicon dioxide deposits with alarge tensile stress in an atmospheric CVD reactor; but deposits with asmaller compressive stress using vacuum evaporation. The amount ofstress will vary depending on deposition rate, temperature and pressure.The insulative film 20 is then coated with a film 22(b) FIG. 1(g) ofsputtered tungsten having compressive stress, and then a second film22(a) having tensile stress with both films together 22 having athickness of 500 Å and a net stress which is low compared with thestress of each individual film FIG. 1(g)!. Once again, the level ofstress must be carefully controlled because it will affect the tightnessof the coil of the rolled electrode. The stress in the sputteredtungsten may be adjusted by changing the sputtering power and gas(argon) pressure. After deposition of the tungsten films 22, thestructure is coated with an optional 100 Å film 24 of sputtered nickel,aluminum, or silicon dioxide FIG. 1(g)! to protect the tungsten from theetchants of the succeeding processing steps.

Next, the structure is patterned with photoresist to define the contactareas where wire bonds will be made. Aluminum is then evaporated on thecontact surface and lifted off elsewhere to provide the bonding pads 26.(See plan views of FIGS. 2 and 4.) Photoresist is again applied andpatterned to define the pixel areas 28 (FIG. 4). Part of the pixel areaoverlays the release layer and part does not. The patterned structure isthen etched using a nickel or aluminum etch (nitric acid for nickel orphosphoric/acetic/nitric solution for aluminum) for coating 24 and thenintroduced into a freon (CF4) plasma which etches through the tungsten22 and the silicon dioxide 20. The plasma will also etch about 100 Åinto the underlying baked photoresist layers 18' and 16' or in thethicker silicon dioxide layer 14. The structure is then exposed toatomic oxygen at 300° C. in a down stream asher. It is preferred to havea process which removes or etches away the release layer by changing thematerial in the release layer from a solid to a gas form. This is calleda dry etch process. This is to avoid the liquid phase. Since any liquidhas surface tension face which can damage the shutters especially whenthe liquid is being removed. However, in a later embodiment it will beshown that when the release layer is removed in the liquid form theliquid can be removed without damaging the shutters by freeze drying. Inseveral minutes the oxygen removes the release layers 18' and 16' andthe moveable electrode 30 FIG. 1(i)! coils up. The structure aftercoiling is shown in FIG. 1(j). Coiling occurs because the bottom silicondioxide 20 has low stress, the first tungsten layer 22(b) hascompressive stress and the second tungsten layer 22(a) has tensilestress and the nickel film 24 has low stress. The structure is now readyfor packaging in a hermetic package and for making the wire connectionsat the edge of the electrodes.

The corrugations provide the lateral stiffness to keep the film fromcurling perpendicular to the intended roll direction. The periodicityand depth of the corrugations are important parameters in thefabrication of the display structure. There may be advantages in makingthe corrugations very deep. Deeper corrugations make the film moreflexible and this would allow the use of thicker, stiffer materials tobe used in the film. A thicker film may be stronger. Another advantageof deeper corrugations would be lower reflectivity of incident light.Light passing to the bottom of a corrugation has a low probability ofbeing reflected back out, especially if the material in the film issomewhat light absorbing.

Instead of using corrugations for anisotropic stiffening, a depositionprocess, such as evaporation or ion bombardment at an angle, may be usedto induce anisotropic stress.

Note that in the process just described, the size of the coil, thevoltage required to roll it out, the holding voltage, and the responsetime are important parameters in the design of particular devices.Assuming other parameters are held constant, some general rules can begiven for these parameters. The larger the roll, the less voltagerequired to unroll the coil and the lower the holding voltage. Thethicker the release layer, the higher the voltage required to roll outthe film. The thicker the insulating layers, the higher the roll outvoltage and the higher the holding voltage. The higher the differentialstress in the two layers, the smaller the coil. The greater the adhesionbetween the moveable electrode and the fixed electrode wherever theymake contact during roll out, the lower the holding voltage. The rollout response time decreases as the roll out voltage increases.

Use of light weight or less dense moveable electrode material results infaster response. For example, use of graphite and diamond ordiamond-like films instead of tungsten and silicon dioxide would producea much faster opening and closing of the valve.

It is also possible to build the shutter so that the insulation filmover the release layer is compressive and the metal film over theinsulation film is tensile. When released this double layer willfunction in a way similar to the structures made using two oppositelystressed tungsten films. An example would be compressive silicon dioxideand tensile magnesium.

Note also that deposited films generally have internal stress which willvary in direction and magnitude depending on deposition conditions. Forexample, evaporated pure titanium is compressive at thicknesses below1000 Å. Adding oxygen to the film changes the stress to tensile. Also,sputtered tungsten deposits compressive at high power and 10 milliTorrargon pressure, and deposits tensile at low power and 15 milliTorr argondepending on the deposition system used. Some films have uniform stress,some films have stress which varies throughout the film. By choosing thematerials and deposition processes, one can generate structures wherethe first part of the film down is compressive and as depositioncontinues the stress gradually becomes tensile so that upon release, thefilm rolls up. Or one can form a single film in which the stress in thelower part is compressive and in the top part is tensile.

Ribbed ELVD Process

An alternate embodiment will now be described with the aid of FIGS.5(a)-5(j). In this embodiment, ribs are formed on the moveable electrodein place of corrugations to produce anisotropic stiffness. The firstpart of the fabrication procedure is the same as in the corrugatedprocedure previously described, including the choice of substrate 10 andcoating and patterning with ITO 12 FIG. 5(a)!. The ITO 12 is coated withsilicon dioxide 14,50 Å to 5000 Å thick, deposited with a CVD system.The silicon dioxide 14 is removed from the edges of the structure wherethe wire contacts will be made using photolithography and etching. Next,the structure is coated with photoresist 16 having a thickness around3000 Å using conventional techniques. The photoresist is patterned,again using conventional techniques, leaving resist in areas whererelease in an overlying film is required. The structure with resist isthen baked at 400° C. for one minute. The resist layer 16' FIG. 5(c)provides the release layer for an overlying film. The patternedstructure is then coated with a 300 Å thick film of CVD depositedsilicon dioxide 42. The glass plate is then coated with two layers 40/41of sputtered tungsten, the first film 41 having compressive stress, thesecond 40 having tensile stress. Since the amount of stress will effectthe tightness of the coil of the micro shutter, the level of stress inboth films must be carefully controlled See FIG. 5(d)!. Next, thestructure is coated with photoresist having a thickness around 6000 Åusing conventional techniques. The photoresist 44 is patterned FIG.5(e)! with a 4 micron period grating with lines running perpendicular tothe roll direction, again using conventional techniques, leaving resist44 in areas where the ribs are desired. The structure with resist isthen baked at 400° C. for one minute FIG. 5(f)! and is coated with 300 Åevaporated silicon dioxide 46 FIG. 5(g)!. After deposition of the oxide46, the structure is patterned (not shown) with photoresist to definethe contact areas where wire bonds will be made. Aluminum is thenevaporated on the structure and lifted off except where required toprovide the bonding pads. Photoresist is again applied and patterned todefine the pixels FIG. 5(h)!. Part of the pixel area overlays therelease layer and part does not. The patterned structure is then etchedusing a freon plasma which etches through the silicon dioxide 46. Thestructure is then etched in oxygen plasma briefly to remove the bakedphotoresist 44'. The tungsten 41/40 and silicon dioxide 42 are etched ina freon plasma etch. The plasma will also etch about 100 Å into theunderlying baked photoresist layer 16' or in the thicker silicon dioxidelayer 14. The structure is then exposed to atomic oxygen at 300° C. in adown stream asher. In several minutes, the oxygen removes the releaselayer 16' and the pixels coil up. The coiling occurs because the bottomtungsten layer 41 is compressive, while the top tungsten 40 is tensileand the silicon dioxide 42 is tensile, but much more flexible than thetungsten. The ribs 46 provide the lateral stiffness to keep the filmfrom curling perpendicular to the axis of the roll. The structure is nowready for packaging in a hermetic package and for making the wireconnections at the edge of the array.

Packaging

The shutters 30 can be strongly affected by water, moisture, and watervapor. The vapor sticks to the surface of the shutter and in turn cancause the shutter to stick down. It has been noted that the shutters runlonger without sticking down when they are operated in a pure nitrogenatmosphere. For reliability, therefore, the shutter array 100 should bepackaged, as shown in FIG. 3, in a dry environment such as atmosphericpressure nitrogen. The preferred seal for the package is a hermetic seal51 which can be made with solder, welding, or solder glass. There aresome organic materials which can provide a good moisture barrier and maybe sufficient as the sealant. One such sealant is butyl rubber.

Preferably, the array 100, as shown in FIG. 3, is enclosed by a cover50, made, for example, of quartz or glass.

Arrays

Most of the applications for the display require having rows or arraysof devices. Television requires hundreds of rows and columns. In FIGS.2-4, a 5×5 array or matrix 100 is shown to illustrate a preferredstructure. In this array, the fixed transparent conductor layer 12 ispatterned into horizontal stripes 52 and the individual moveable shuttercoils 30 are connected together in columns 54 overlying the horizontalstripes. Each of the array pixels is therefore at the junction of onehorizontal and one vertical electrode. In the rolled up position, acertain fraction of the light impinging on one side of the plane of thearray is blocked by the rolls. For example, if the roll diameter is 40microns, and the overall pixel size is 130×130 microns, then about 30%of the light is blocked by the rolls. When all the shutters are in therolled out position, as shown in FIG. 4, there are small gaps betweenthe shutters to provide some clearance. Light can pass through thesegaps unless additional opaque stripes are provided in the substrate toblock these openings. If the gaps are 4 microns wide, then withoutadditional masking on a 130×130 pixel size, a 20 to 1 contrast ratiowould be expected, assuming opaque shutters. With additional masking,much higher contrast ratios are possible. Also, a larger pixel size withthe same roll diameter will give a larger contrast ratio.

Color Displays

Light may pass through the display in either direction and a colordisplay could be made by using color filters, as shown in FIG. 6,wherein a display array 100' is formed, as previously described, andpackaged in a glass or quartz hermetically sealed container comprised ofbottom glass substrate 10, sidewall spacers 80 and glass or quartz topwall 60. Red, green and blue filters 72, 74, 76, respectively, arebonded to the bottom of wall 60 and light is passed in the direction ofthe arrows. A second version of the color display is shown in FIG. 7,where ultraviolet light is supplied to one side of the display 200 andthree ultraviolet light sensitive color phosphors 84, 86, 88 (red, greenand blue, respectively) are positioned on the other side beneath wall 60to produce an active display. Each coiled electrode 30 controls onecolor phosphor. This approach has the advantage that the amount offlicker can be controlled by the persistence time of the phosphor. Toenhance the contrast of this display, an array of collimating orfocusing lenslets 90 on the ultraviolet or light source side of thearray, may be provided as shown in FIG. 8. These would serve to focusthe light from a source 91 onto an aperture 97 through a shutter 30 ontoits respective phosphor or respective filter 72', 74' or 76'. Lightabsorbing barriers 93 could also be used to enhance contrast and reducecrosstalk. The design of these lenses could take advantage of the workin the field of binary optics. The array could also be used in thereflection mode, using a white background 95 and black shutters 30, asshown in FIG. 9. An even simpler reflection mode device may be madeusing an opaque white conductor 12', instead of the transparentconductor and eliminating the white reflective layer 95 behind the glasssubstrate 10. In a very advanced version, two shutter arrays A and B arestacked, as shown in FIG. 10, to produce a passive color display 500.The top array A has black shutters 30', the intermediate array has threecolors on the shutters (blue, green and red) 30b, 30g, 30r and thebackground 95' is white. This provides a multicolor reflective displaywhich is totally passive.

Voltage Waveform

The power requirements for the display device of the invention are verysmall. The voltage required is in the range of 1 to 50 volts. There isessentially no DC current required. Most of the current is the verybrief flow of charge as the shutter or coil closes or opens. Thiscurrent charges the capacitance between the moveable shutter electrodeand the fixed electrode. The power required to close a shutter having a364×130 micron pixel area 30 times a second is about 3×104⁻⁸ watt. Ahigh definition screen having 1000 rows and columns in color would havethree million pixels. A 14×16 inch, 21 inch diagonal color display at aframe speed of 30 cycles per second, represents a power consumption forthe entire screen of about only one-tenth of a watt. A representativewaveform of the voltage across the two electrodes of the shutterrequired to turn on (roll out), hold, and turn off (roll up) theshutter, is shown in FIG. 11. When the voltage moves above the turn-on(or roll-out) threshold, the film electrode rolls out, and can be heldout by a lower voltage. The reason the holding voltage is lower than theroll-out voltage is that a step is provided on the moveable electrode aswill be explained in connection with FIGS. 24 and 25. Thus, for a givenvoltage on the two electrodes the electric field and, hence the forcebetween the two electrodes, becomes larger as the coiled electrode movescloser to the transparent electrode. Therefore, once the moveableelectrode is rolled out, the voltage can be reduced and still retainenough field to hold the electrode down.

Under some circumstances, it may be necessary to use an AC voltage tohold the electrode down, as shown in FIG. 12. The frequency of the ACcould vary widely depending on device design. A frequency of one cycleper second has worked well for minimizing drift, for some devices whichhave been fabricated. This is required if charge migration occursthrough the two insulating films 14 and 20 See FIG. 1(h)!, which eithercauses the film to stick down or causes the threshold to shift. Usingthe AC voltage can increase the power consumption for some applications,but there will be substantially no increase for applications such as TVwhere the AC frequency used is 30 cycle/second or lower.

In a matrix, the bistable nature of the device can be used to greatadvantage. An example is given in FIG. 13 of the voltages used on thehorizontal (h) and vertical (v) lines to switch individual pixels. Thegeneral idea is to maintain a bias voltage between the two coiledelectrodes at every pixel which is halfway between the turn-on thresholdand the turn-off threshold. This bias will hold whichever state thepixels are in. If one of the horizontal lines is raised in voltage,raising the voltage between the electrode pairs in that row, to justbelow the turn-on threshold, none of the previously Off pixels willswitch On. If, however, at the same time, one of the vertical lines islowered in voltage, which raises the voltage between the electrodes inthat column, to just below the turn on voltage, none of the previouslyOff pixels will turn On, except the one at the intersection of verticaland horizontal lines. FIG. 13 shows three scans of the entire matrix,turning on or Off several pixels in each scan.

Projection System

Any of the shutters or shutter arrays previously described may be usedin a projection system, as shown in FIG. 14, wherein light is focused ona matrix array of the invention 100 and an image is formed which isprojected by projection lens 200 onto screen 300. Since the pixels arenot very heat sensitive, a large amount of heat can be dissipated orreflected and therefore high-intensity light can be handled andprojections onto very large screens is possible.

For proper operation of the electrostatic light valve displays of theinvention, it is important to take into account the effect of the smallDC current in addition to the capacitive charging current, when avoltage is applied across the electrodes. This current exists eventhough the electrodes are separated by one or, preferably, twoinsulators. The term insulator is misleading in this context, becauseone generally thinks of electrical insulators as non-conducting. In thepresent invention, a minimal current, as will be explained later, ispresent and can be desirable between the moveable electrode and thefixed electrode to minimize power dissipation. For this reason, completeblocking of current is not required. Instead, the "insulators"previously described are actually formed of high resistivity materialswhich are used as part of the electrode in a way which reduces chargeflow between the electrodes. These materials can have resistivities(which vary depending upon applied voltage and temperature) in the rangeof that of silicon dioxide, i.e., 10¹⁰ -10¹⁵ ohms-cm. or they can haveresistivities considerably lower and still function to reduce the chargeflow sufficiently.

Design Considerations

Some of the electrode design considerations and factors will now bediscussed in connection with FIG. 15, in which the structure of therolled moveable electrode is shown as a flat electrode for simplicity ofillustration. Note also that this discussion is based on a simplifiedtheoretical analysis based on observation of experimental structuresmade in accordance with the invention. Other factors may be involved andthe validity and usefulness of the present invention should not bepremised on the accuracy of this theory.

In general, if only one high resistivity layer 114, on either themoveable 130 or fixed 120 electrode is used, as shown in FIGS.15(a)-15(c), it is difficult to keep the electrode in the "rolled-out"position. During roll out, FIG. 15(a), the voltage provides a force dueto the accumulation of charge on each electrode. As soon as theelectrodes touch or come within about 100 Å, FIG. 15(b)!, however,charge Q can transfer from the metal to the surface of the highresistivity material 114, filling some of the surface states, and theforce is reduced FIG. 15(c)!, and the electrodes can move apart orroll-up. This charge transfer can happen rapidly so that the moveableelectrode will barely begin to roll out before it rolls back up. Ingeneral, a single sheet of insulator 114 will not work because themoveable electrode 130 will not roll out and stay rolled out.

However, if the surface roughness between contacting surfaces issufficiently large and keeps the two surfaces sufficiently far apart(>100 Å) over a sufficiently large area to block a large part of thecharge transfer, the device can be made to operate and stay rolled out,since an electric field can exist across these air gaps and provide aforce to hold down the rolled out electrode. The voltage required toroll out the shutter when there is an air gap will be higher than for aninsulator without an air gap for the same conductor separation becausethe dielectric constant of the insulator is higher than the air gap.Thus, the disadvantage of the air gaps is that they tend to raise theoperating voltage of the device. The problem of reduction of electricfield due to charge transfer can also be avoided by using an AC, ratherthan a DC voltage, to roll out and hold out the moveable electrode.Keeping the roll out flat requires a high enough frequency so that thefield reverses after only a small amount of charge transfers and theroll does not roll back up to any significant extent between each fieldreversal. The AC mode of operation has the disadvantage that it requiresmore power for operation than the DC mode and may tend to cause morewear because parts of the roll may tend to oscillate at twice theapplied frequency. Any of the metal atoms removed from the moveableelectrode, due to wear, will very likely be left behind on the highresistivity material adding to the surface charging effects.

Dual Radius Shutters

Up to this point, all of the shutter electrodes are shown to have aconstant radius so that when they are rolled up, they form a cylinder.If the shutter 30' is long enough or if the radius of the cylinder issmall enough, the coil electrode 30 will roll beyond a single turn andcome in contact with itself. This could cause a reliability problem,because there could be places where the electrode is rubbing againstitself and chafing and wearing. This problem can be overcome by having ashutter which has a variable radius of curvature, as shown in FIG. 16,wherein a glass substrate 10 supports fixed electrode 12 beneathinsulator 14. Moveable electrode 30' has a small radius R1 near thesubstrate attachment and a much larger radius R2 farther out. In thisdevice, the large radius R2 provides for a long roll-up distance withoutself contact. In the second device of FIG. 17, the larger radius R2 isnear the substrate attachment, and the smaller radius R1 farther out.This second device also has a long roll-up distance, but rolls up in aspiral shape to avoid self contact.

The above may be realized in practice, as shown in FIG. 18, by forming atrilevel shutter 30' in which the moveable electrode is formed of ametal electrode 22 sandwiched between two insulators 20 and 24 (as willbe described in detail later) The top insulator 24 is partly removednear the attachment area 40, so that when the moveable electrode 30rolls up, it has a dual radius with the larger radius R2 remote fromarea 40 and the smaller radius R1 closer to area 40.

Rail Embodiment

FIGS. 19(a) and 19(b) illustrate an embodiment wherein much of the highresistivity material has been eliminated. Standoffs or rails 420 made ofSiO₂, or other high resistivity material, support the moveable electrode130 when it is in a rolled down position FIG. 19(b)! so that a gap isformed between electrodes. The gap is filled with gas or vacuum. The airgap in this case is uniform and carefully controlled and results in acontact plane with almost no surface state charging at the interface,resulting in a reduction of the charge problems associated with a solidinsulator. The standoffs 420 may consist of narrow ribs of siliconnitride or silicon dioxide running perpendicular to the roll direction.To provide better damping, the insulating ribs, could also be made outof plastic which has greater flexibility than silicon dioxide or siliconnitride. To reduce the charge problems attributed to the solidinsulators even more, one could coat the tops of the solid insulators420 with metal 422 FIGS. 19(a), (b)!. The metal would then charge up tothe potential of the moving electrode 130 each time it rolled out, whichwould eliminate the drift in voltage. The metal on top of the rail couldalso be permanently electrically connected to the moveable electrode.One could also have extra high resistivity material between the railsand/or on the moving electrode for some applications. Note that materiallocated between the rails must be thinner than the rails in order tomaintain a gap.

An alternate geometry for the stand-off embodiment is shown in FIGS.20-23 in which the rails 420' run in the roll direction along the fixedelectrode 112 over the glass substrate 110. In this geometry themoveable electrode 130 is planar on the fixed electrode side in order tohave the roll out motion smoother and have less friction.

For many applications, it is desirable to have a bistable displaydevice, that is, one in which the "hold-on" voltage is less than thevoltage required to initially turn the shutter on. This is especiallyuseful in x y scanned arrays, which would be used to make a televisiondisplay screen, for example. This bistability can be created by forminga step S in the moveable part of the moveable electrode 30 (formed ofstressed conductor 22 and insulator film 20) near the place where it isbonded to the insulator 14 on the fixed electrode 12, as in FIG. 24. Thestep S increases the distance "x" between the electrodes which increasesthe voltage required to begin to roll out the electrode. The amount ofvoltage required can be controlled by the step height. Once the coilbegins to unroll, as in FIG. 25, the distance "x" between electrodes isless, so that less voltage is required to roll out the electrode. It is,therefore, possible to use a pulse, as previously shown in FIG. 11, tounroll the coil where the initial part of the pulse has a larger voltagewhich drops down for the rest of the pulse. The minimum voltage requiredto begin the unrolling is Vt1 and the minimum voltage required tocontinue the unrolling is Vt2. As long as the voltage between theelectrodes stays above Vt2, the coil will stay most of the way rolledout. For any voltage between Vt2 and Vt1, the coil position will movevery little, either in the rolled up or rolled out position. If thevoltage is at any point dropped below Vt2, the coil will roll up. Onecould, of course, introduce additional steps along the coil which wouldrequire exceeding threshold for that step before the coil could rollbeyond that step.

A second way to create a bistable device is to use the naturalattractive adhesion forces (Van der Walls Forces) which occur when twomaterials come into contact. The amount of attractive force depends onthe material and the surface smoothness. By selecting materials andcontrolling the surface condition a magnitude of adhesion force can beachieved which is low enough to allow roll-up of the shutter at zeroapplied voltage but large enough to significantly reduce the holdvoltage below the roll out voltage, assuming there is no step. Theresult, as with the case for the built-in step, will be bistabilitywhere the shutter can be open or closed of the same voltage. It is evenpossible that the adhesion forces can be used to provide stability to apartially rolled out shutter, as will be described later.

Grey scale can be provided by partially rolling out the shutter or byusing time multiplex and the natural persistence of the human eye togive the appearance of gray. For example, in an application for TV, theframe time is 1/30 of a second. Since the shutters switch very rapidly,they can be turned on and off in 1/10,000 of a second or faster. Byleaving a shutter on for only a fraction of the frame time, theappearance of grey scale an be achieved at each pixel.

Other methods for achieving grey scale will be described in the latersections.

Passive Multicolored Bistable ELVD

In the embodiment of FIGS. 26a and 26b a passive multicolored bistableelectrostatic light valve display is depicted wherein a plurality ofcoiled moveable electrodes 30a-30d of the type previously described aresymmetrically disposed about a central display area 510.

The moveable electrodes 30a-30d are each affixed at one end to insulatorlayers 14 formed on white electrical conductor 12 formed on aninsulating substrate 10.

Individual moveable electrodes 30a-30d may be selectively activated, oneat a time, by applying voltage from source V across selected electrodesto vary the color of the light reflected back to a viewer on thesubstrate side. Arrays of such displays may be provided to produce apassive color display.

Deformable Membrane Switch

An alternate embodiment will now be described in connection with FIGS.27(a) and 27(b) which shows a deformable membrane switch (DMS) 900operating in the reflective mode in schematic cross-section form. Amembrane 902 is shown curved upward in the "up" position. Membrane 902is formed of a transparent conductor 901 disposed on top of an insulator920 and bonded at its two lateral edges to an optional insulator support916. Support 916 is disposed on insulator 914 which is formed on opaqueconducting layer 912, deposited on glass substrate 910. In this "up"position, membrane 902 can be made to look white or the color ofconductor layer 912 when viewed from above. In FIG. 27(b), a voltagei.e. 20 volts is applied across the electrodes 902 and 912 causing mostof the membrane 902 to lie flat against the insulating layer 914 andlook blue or black from the top because electrode 902 forms anantireflection coating on the opaque conducting layer 912. Membraneelectrode 902 is biased in the upward position by anisotropic stress oranisotropic stiffening using the principles set forth previously withrespect to the coilable electrode. As with the previous embodiments, theoperation of the deformable membrane switch 900 depends on a voltageapplied to two electrodes. One electrode is the membrane 902, the otheris the conducting layer 912. Starting with zero volts applied themembrane is in the "up" position and has an upward curvature due to thestresses or stiffening built into the membrane film or because it wasfabricated with that shape. When the voltage is above the turn-onthreshold, the membrane moves down into contact with the insulatingsurface 914. Once the membrane is in the down position, the voltage canbe lowered and the membrane 902 will remain in the "down" position, aslong as the voltage remains above the turn-off threshold. As with theprevious embodiments, the DMS is a bistable device. This device can alsobe used in the transparent mode with a transparent substrate where themotion of the membrane causes a change in color due to dielectricconstant differences and interference effects. A second version is shownin FIGS. 28(a) and 28(b). In this version, the device can be illuminatedfrom the back. An array of opaque stripes 940 is formed on insulator 914on a transparent lower electrode 912 and a second array of opaquestripes 930 is formed on the moveable electrode 902. The arrays arestaggered so that the stripes on the upper electrode 902 fit in the gapsof the lower electrode 912. With the membrane 902 in the up position,light can pass through the valve by diffraction and reflection throughthe slits, and with the membrane in the lower position, light issubstantially blocked. The membrane 902 and other elements of the DMSmay be formed of materials and using processes previously described inconnection with the ELVD embodiments.

Dry Roll-Up Process for Forming ELVD

Referring now to plan views FIGS. 29(x) and 29(y) and sectional viewsFIGS. 29(a)-29(t), an alternative dry roll-up process for fabricating anarray of electrostatic light valves for an ELVD will be described indetail. Note: For simplicity, fabrication of only a single ELVD is shownin these figures. It should also be understood that a linear or twodimensional array can be made in the same manner. In step 1 FIG. 29(x)!,a transparent layer coating 12 of, for example, indium tin oxide (ITO)is formed to a thickness of about 2000 Å on a suitable substrate 10,made, for example, of glass. Next, the structure of FIG. 29(x) isovercoated with photoresist (not shown) which is patterned using aconventional lithography process. The exposed ITO 12 is then etchedusing the photoresist as a mask. The resist is then removed leaving alower fixed electrode 10(a) separated by strips from adjacent fixedelectrodes, as shown in FIG. 29(x).

Next, as shown in FIG. 29(a) which is a section taken through lines 29'--29' of FIG. 29(x), an insulator layer 14 of, for example, aluminumoxide (Al₂ 0₃) is formed over the ITO coating 12 to a thickness of about2000 Å. An optional thin layer of polytetrafluoroethylene (PTFE) may beformed over layer 14 to enhance inner stress and to lower adhesion.

The purpose of the PTFE layer is to control adhesion forces which, inturn, controls the bistability, as described earlier. One couldsubstitute other materials for the PTFE to achieve the desired adhesionlevel.

As shown in FIG. 29(b), a release layer 16 comprised of material, suchas silicon or germanium which is susceptible to preferential dry etchingis formed over the insulator layers 14/15. The thickness of layer 16will depend upon the design parameters of the light valve, but athickness of about 5000 Å is normal.

In step 3 FIG. 29(c)!, a photoresist pattern 18 of the type described inconnection with FIG. 1(b) is formed over layer 16 using conventionalmethods. Pattern 18 is used as a mask to define the release areas. Thisresist pattern 18 is baked at about 170° C. Then a second layer 18' ispatterned in the shape of a grating over the first resist area 18.

In step 5 FIG. 29(e)!, the sample is placed in a reactive ion etcherwhere conditions are established such that the photoresist regions18/18' are etched at about the same rate or a little faster than thesilicon layer 16. For example, using a CF₄ gas at 10 millitorr at anetching voltage of 500 volts, reproduces the photoresist profile in theremaining silicon layer 16' to form rounded corrugations 17. It isdesirable to have the rounded shapes of the baked photoresist tominimize high stress lines in the resulting moveable electrode. A topview of the structure of FIG. 29(e) is shown in FIG. 29(y).

In step 6 FIG. 29(f)!, the wafer is coated with an optional layer 15' ofPTFE, which for simplicity is shown only in FIG. 29(f). Once again, thepurpose of the PTFE is to control adhesion forces to control thebistability as described earlier. One could substitute other materialsfor PTFE to achieve the desired adhesion level. The wafer is then coatedwith aluminum oxide about 300 A thick having compressive stress. Acompressive stress aluminum oxide coating is achieved by sputteringusing an aluminum oxide target. Compressive stress occurs under normalsputtering conditions when the material is sputtered using Argon as thesputtering gas at 10 Mtorr pressure, 100 watts power using a 5" target.This is followed, in step 7 FIG. 29(g)!, by a low-stress layer 22 ofaluminum having stress much lower than the aluminum oxide. In step 8FIG. 29(h)!, a second layer 24 of aluminum oxide is formed to about 300Å having tensile stress. Tensile stress is achieved by adding a littleoxygen under the same conditions. Test runs with evaluations of thestress can be used to establish the proper conditions for achieving thecorrect stress in the coating process using, for example, sputtering orevaporation. The stress measurement procedure is given in "TheMeasurement of Internal Stress in Electrodeposits", D. R. Cook and J. M.West, Transactions of the Institute of Metal Finishing, 1963, Vol. 40.One of the goals of establishing correct stress in the deposited layersis to provide a symmetrical structure, so that temperature changes willhave minimal effect on the roll size. Since the aluminum expands at20×10⁻⁶ /° C. versus around 6×10⁻⁶ /° C. for aluminum oxide, theinternal stress of the moveable electrode will vary with temperature.However, since there are equal thicknesses of aluminum oxide on bothsides of the aluminum layer, temperature changes will not cause thecoiled electrode to roll out or up.

Preferably, the top surface should be opaque or black and the bottomsurface reflective, therefore, an additional layer 26 is provided atthis point FIG. 29(i)! to produce a black surface. Layer 26 may becomprised of carbon black containing very fine particles, less than 1500Å in diameter, which can be made into a dispersion in a solvent andresin. This can be used as a coating which is spun on like photoresist,or sprayed on. The material is very flexible having a low Young'smodulus. Therefore, as long as it is not too thick, it will not have asignificant effect on the mechanical properties of the moveableelectrode.

At this point, the moveable electrode 30, formed of layers 20,22,24 and26, may be patterned using photoresist as a mask and using a combinationof reactive ion etching for the carbon black layer 26 and the aluminumoxide layers 20 and 24 by chemical etching the aluminum layer 22. Theoptional PTFE layer 15' may be removed by Reactive Ion Etching (RIE).The wafer is then placed in a plasma etching system in a fluorine orhalide bearing gas. The halogen gas, without the plasma, can also beused to etch away the silicon, releasing the moveable electrode 30. Thepatterned FIG. 29(j) structure is shown in the top view of FIG. 29(z)before the electrode has coiled and after coiling in the top view FIG.29(z'). FIG. 29(j) is a section taken along lines 29'--29 ' of FIG.29('). FIGS. 29(k) and 29(l) are side-views as in FIG. 29(j) in anenlarged scale and schematic form showing the completed electrodestructure before the silicon release layer 16' is removed (FIG. 29k) andafter the silicon release layer 16' is removed (FIG. 29l).

Low Cost Shutter Fabrication

Next, a low cost method of shutter fabrication will be described withthe aid of FIGS. 30(a)-30(p) and FIGS. 36(a) and 36(b). In this process,we begin with a transparent substrate 10, upon which is formedsuccessively a conductive, transparent electrode film 12 (e.g., ITO), aninsulator film 14 (e.g., aluminum oxide) FIG. 30(z)!. A release layer 16of silicon is formed on the insulator 14 FIG. 30(b). In FIG. 30(c) aresist layer is formed on the release layer 16 and patterned into agrating 18 which will be used to produce corrugations in the surface ofthe silicon layer 16.

In FIG. 30(d) the sample is placed in a Reactive Ion Etch (RIE) chamberand etched in CF4 gas until the resist is barely etched away leavingsilicon corrugations 16' formed in the silicon layer but withoutpenetrating through the layer 16. In FIG. 30(e) the sample is coatedwith successive layers of aluminum oxide 20, aluminum 22 and aluminumoxide 24 as described in the previous embodiment. These three layerscontain the necessary stress to form an electrostatically rotatableshutter 30 when released from the substrate.

FIG. 30(f) is the same as FIG. 30(e) except that the shutter 30 has beenshown in simplified form as an integral structure with the addition ofoptional layer 15 as described in the previous embodiment.

Next, the structure of FIG. 30(f) is patterned with photoresist 34 toform stripes only, for the column address lines. If only stripes arerequired to be patterned, a very low cost lithography tool can be used,such as a scanned laser, or laser holography method, instead of the moreconventional and very expensive mask aligned or stepper method requiredfor forming more complex patterns.

As shown in FIG. 30(g), stripes or grooves 32 for the column addresslines are formed by etching through to the ITO layer 12 using acombination of RIE and chemical etch using photoresist 34 as the mask.

In the next step FIG. 30(h)!, the ITO is etched so that it undercuts at44 the silicon dioxide. The reason for this is that in the next step (FIG. 30(i)!, a black or opaque layer 40 is formed in the opening wherethe ITO was removed and the black layer is almost always somewhatelectrically conducting. By undercutting at 44 an insulating gap isprovided to insulate between the column address lines and the blacklayer.

In the step shown in FIG. 30(i), the wafer is placed in a vacuumevaporator and chromium is evaporated in the presence of oxygen whichyields a cermet layer 40 which is black or has a dark gray appearance.The chromium must be evaporated so that the evaporating beam is fairlywell collimated perpendicular to the sample in the directionside-to-side relative to the sample shown in FIG. 30(k). This can beaccomplished by keeping the sample perpendicular to the beam andevaporating through a slit parallel to the slots 32 etched in thesample. An aluminum oxide layer 42 is then evaporated in the same way.The object is to make the overall thickness of these two films 40/42about the same as the overall thickness of the original underlying ITOand aluminum oxide films 12/14 plus optional layer 15. There willinevitably be a small amount of deposition on the side walls of theslots 32 which can be removed in a short wet chemical or gas cleanupetch.

Next FIG. 30(j)!, the photoresist layer 34 is dissolved in a photoresiststripper. The photoresist 34 therefore forms a release layer for themetal on top of it which is not needed on the final product.

At this stage, the column patterning of the ITO layer 12 and themoveable electrode 30 is complete, as shown in the top view of FIG.30(k) and the sectional views along lines 30'--30 ' of FIGS. 30(d) and30(j). The row address lines can now be formed as illustrated by the30"--30" cross-section of FIG. 30(l) and 30(m). In the step of FIG.30(m), two resist layers 50 and 52 are formed in such a way that thelower layer 50 is undercut 62 relative to the top layer. Such a doublelayer could be formed, for example, by an Az 1370 photoresist on top ofPMMA. One could also use a patterned metal layer on top of a photoresistlayer. The purpose of this double layer is to provide an undercutrequired for lift-off later, after shadow evaporation in the next step.This multiple resist layer 50/52 is then used as the etch mask to etchthrough the movable electrode layer layer 30 and the silicon layer 16,stopping at the PTFE 15 or aluminum oxide layer 14 to form row grooves60.

Next, as shown in FIG. 30(n), the wafer is again placed in a vacuumevaporator and aluminum is evaporated at an angle this time where theevaporant beam is collimated as before to form aluminum regions 70a and70b. The thickness of the aluminum layer is preferably around 2000 Å.The angle evaporation of region 70b provides the electrical connectionin the row grooves between shutters and the row lines 80 FIG. 30(p)!extending to the edge of the sample. The aluminum is also the mechanicalsupport for the coil after it has rolled up. This is significantlydifferent from other embodiments where the shutter coil 30 was attacheddirectly to the substrate insulator.

In the step shown in FIG. 30(o), the photoresist 50/52 is dissolved, andsince the metal layer 70(a) is discontinuous, as provided by theundercut resist layer 50, the unwanted metal layer 70(a) will float awayin the solvent. A top view of the structure to this point is shown inFIG. 30(p).

The wafer is now ready for a fluorine etch to remove the underlyingsilicon 16 FIG. 30(o)! which will allow the moveable electrode 30 toroll up.

It should be noted that the shutter or moveable electrode 30 formed inFIGS. 29(a)-29(l) and 30(a)-30(f), differs from previous embodiments inthat the electrode 30 is of three layer construction having alternatingmaterials. As shown in FIG. 36(a) before release! and 36(b) afterrelease!, electrode 30 is formed of a first (lower) compressiveinsulative layer 20 (as indicated by arrows 90), a second intermediatemetallic layer 22 having low or zero stress and a third (upper)insulative layer 24 formed with tensile stress (as indicated by thearrows 92). The structure of moveable electrode 30 has severaladvantages over previous embodiments. The net degree of stress in thecomposite/laminated structure 30 is easier to control, the tri-levelelectrode can be made more optically dense and more flexible. Also, aswill be shown below, it can be fabricated by a low temperature process.

Process for Fabrication of ELVD Shutters Using Freeze Drying

In this section, an alternate low temperature method of shutterfabrication will be described with the aid of FIGS. 31(a)-FIGS. 31(i).In this process, we begin with a glass substrate, as was describedabove, having an ITO layer 12 forming a transparent fixed electrode.Stripes are etched as described in previous embodiments. An insulatorlayer 14 followed by an optional PTFE layer 15 (shown only in FIG. 31(a)for simplification). In this example, silicon dioxide is for theinsulating layer 14. Polished ceramic could be used as the substrate 10instead of glass.

A pattern of photoresist is used to provide a release layer 18 ( FIG.31(b)!. The resist is double-exposed, as shown in FIG. 31(c), to providea corrugated electrode structure 19. The first exposure provides therelease pattern and the second exposure, which is made through a 4 μmgrating, is a much lighter exposure. When combined with a carefullycontrolled development, the photoresist can be removed in the areasexposed to light, but only half-way through the resist. This procedureresults in the corrugations 19 shown in the resist layer 18. Also, theresist is baked at lower temperatures of about 120° C., so that it willstill be soluble in a solvent, such as acetane.

In FIG. 31(d), the wafer surface is coated with an optional layer ofPTFE (not shown) which can be provided using sputtering. Then a firstcoating of silicon dioxide is formed to provide an insulative layer 20having a thickness of around 300 Å and having a compressive stress. Asbefore, test runs with evaluations of the stress can be used toestablish the proper conditions for achieving the correct stress in thecoating process using, for example, sputtering or evaporation.

In FIG. 31(e), a coating 22 of aluminum having low stress is formedusing an evaporation or sputtering process.

In FIG. 31(f), alternating layers consisting of silicon dioxide andchromium are deposited starting with the silicon dioxide to form a thirdcoating 24. The silicon dioxide is around 100 Å thick, and the chromiumis around 50 Å thick. The silicon dioxide in this series of coatings 24is made to be low stress and the chromium is made to be tensile. Thechromium layers are light absorbing and make the surface almost black asin chrome black. The more coatings the darker the color.

In the next step FIG. 31(g)! photoresist (not shown), is applied andpatterned in the conventional manner in order to provide a mask foretching the resist 18 underlying the shutters 30. The resist 18 can thenbe etched using a combination of RIE and a wet chemical aluminum etch.

In FIGS. 31(h) and 31(i), the cross-sectional scale has been reduced forsimplification. FIG. 31(h) shows the structure before release layer 18has been removed. FIG. 31(i) shows the structure after the wafer hasbeen placed in acetone to remove the resist whereupon the membranes 30roll up. The sample is then gently rinsed in water and the sample andthe water surrounding the sample is cooled until the water freezes. Thewafer, which is now embedded in ice, is held at a temperature of around-10° C. and is placed in a vacuum chamber at a pressure of a few Torrwith an optional nitrogen flow. The ice evaporates after 15 or 20minutes which completes the fabrication process. The freezing step isimportant since it enables the water to evaporate from the solid stateso that the surface tension forces of the liquid does not destroy theshutter which otherwise would occur during drying from the liquid state.

Hinged Shutter for ELVD

Referring now to FIGS. 32(a)-32(i) and FIGS. 33-35, a process forfabricating a hinged shutter for an ELVD will now be described inconnection therewith. The process begins with a transparent substrate 10(e.g., glass) having a transparent conductive coating 12 formed thereon(e.g., an ITO coating). The coating 12 is etched at dotted lines 13 FIG.32(a)! using standard lithography techniques to provide two substrateelectrodes 12(a), 12(b) for operation of the shutter. In the next stepFIG. 32(b)!, the wafer is coated with an insulative layer 14 (e.g., analuminum oxide layer). In FIG. 32(c), the aluminum oxide layer 14 ispatterned with standard lithography techniques to provide an insulatingcontact surface 15' on one of the electrodes 12(b). The wafer is thencoated with a layer of material which may be preferentially dissolved(e.g., silicon) to form a release layer 16 FIG. 32(d)!. In FIG. 32(e),the wafer is coated with a tensile stress layer 18 (e.g., of aluminumoxide). This layer 18 is etched into a rectangular annulus 18' in FIG.32(f) (see top plan view FIG. 33) which because of its shape and stresswill cause a slight bowing of the final hinged shutter so that it has abowl-like shape. This same aluminum oxide layer will also be located attwo of the contact points of the shutter. In FIG. 32(g), the wafer iscoated with an aluminum layer 20 which is patterned FIG. 32(h)! into thefinal shape of the hinged shutter 20' outline (FIG. 34). The releaselayer 16 of silicon is then etched away using fluorine gas FIG. 32(i)!and the hinged shutter settles down to rest on the surface of the waferand is held in place by Van der Wall's forces and makes contact on atleast 3 corners. As shown in the plan view of FIG. 35, two of thecontact corners 30(a), 30(b) are contacts between electrical conductors,and two of the contacts 30(c), 30(d) are between electrical insulators.The conductor contacts will be inherently more sticky than the insulatorcontacts and therefore when a proper force is applied to the shutterwith an electric field, the insulator contacts will break free and theshutter will rotate about an axis A passing through the two conductingcontacts.

Anisotropic Stress Strips

In order for the shutter electrodes to roll only in one direction eitheranisotropic stiffness or anisotropic stress is needed. One approach toachieving anisotropic stress is to form stress strips, as shown in FIG.37. In this embodiment, a trilevel electrode structure 30 is formedwhich consists of bottom layer 20 of low stress Al₂ O₃, and intermediatelayer 22 of low stress Al and a top layer 24 of high stress Al₂ 0₃ usinga sputtering or evaporation process as previously described. Cut lines33 are etched through the top film 24, so that the film is etched into agrating, or series of strips 31. The film 24 becomes very anisotropic,and the stress of the film becomes anisotropic. The film is stilltensile in the direction of the arrows A, having a stress of the sameorder as the original film, however the stress perpendicular to thestrips 31 is much lower. The stiffness will, of course, also beanisotropic, and will be less stiff when rolled perpendicular to theroll direction. However, since the stress is more anisotropic than thestiffness, the stress will dominate the roll direction. The stress inthe film 24 will be isotropic in the plane of the film. The need for thecorrugations is eliminated, in this embodiment, since the main purposefor the corrugations is anisotropic stiffness to control the rolldirection. Instead the stress strips 31, which lie in the rolldirection, provide control of the roll direction.

Grey Scale

For many applications, such as TV, it is important to be able to havenot just an on off, black and white, light valve but a modulator whichcan vary the intensity to have at least 20 and preferably more shades ofgray. Since the human eye is an averaging device which tends to averageover space and time, there are a number of methods for providing grayscale which take advantage of this averaging quality. One approach forproviding gray rather than white is to leave the shutter open only forhalf the time instead of all the time. If the shutter is turned on andoff at least 30 to 60 times a seconds, the eye will average and lightand see gray instead of white. The achievement of gray scale in thisway, by using driving electronics, has been experimentally demonstratedusing the shutters. This approach has certain disadvantages, one ofwhich is that using this mode, the shutter speed required increases withthe number of shades of grey.

A second approach is to open the shutter only half way instead of allthe way. If the pixel is small enough the eye will spacially average and"see" gray rather than just a half open shutter. In a TV application,because there is an array of devices representing a large number ofpixels, the array needs to be scanned a row at a time, and thereforeeach pixel can only be addressed for a small fraction of the total frametime. This means that the device used for each pixel must have a memoryquality which allows it to retain its position while it is not beingaddressed.

A special electrode configuration which will allow the device to belocked in a partly rolled out position will now be described in detailin connection with FIGS. 38-42. FIG. 38 is a simplified side view of anindividual shutter 300 comprising substrate 10, fixed electrode 12,insulator 14 and coil electrode 30. A first cross-section 40--40, wherethe coil electrode 30 is rolled out and in contact with the surface ofinsulator 14, is shown in FIG. 40. A second cross section 41--41, wherethe electrode 30 is not yet down on the surface, is shown in FIGS. 41and 42 in different modes of electrode potential.

The unique feature of this embodiment which allows it to have gray scalecapability is the way the lower or fixed electrode 12 is segmented, asmore clearly shown in the perspective of FIG. 39. Where previously therewas just one fixed electrode for a single shutter 30, the fixedelectrode has now been subdivided into five separately chargeableelectrodes 12(a)-12(e), which run parallel to each other in thedirection of the rolling motion of the shutter 30. When the five fixedelectrodes are connected together electrically and are at the samepotential, the device of FIG. 39 will operate very similar to thedevices previously described. By properly manipulating the potential onsome of the fixed electrodes while the shutter 30 is rolling out, it ispossible to stop the motion of the electrode and hold it, or lock inthat position. It is important to be able to hold the membrane in itslocked position even through the potential on the electrode 30 is variedsomewhat. This is because in an array, with the fixed electrodes as therows, and the moveable electrode as the columns, with all the electrodesin any given row locked in position using fixed potentials on the rowelectrodes, there will be small variations in the potential betweenlimits in the column of moveable electrodes as the other rows are beingaddressed.

FIG. 42 is a cross-section of FIG. 38 taken at 41--41 showing theshutter 30 in the roll out mode wherein all the fixed segment electrodes12(a)-12(e) are at the same potential E1 and the membrane electrode 30is at a different potential E3 whereby the shutter 30 is caused to rollout onto the insulator 14 in a manner similar to the case forunsegmented fixed electrodes.

In order to lock the moveable electrode 30 in place, a high E field isrequired to be maintained between the moveable electrode 30 and thefixed electrode 14 wherever the moveable electrode is in contact withthe fixed electrode, as in FIG. 40 in order to counteract the springforce which is trying to roll the membrane up. The size of the E fieldmust be even larger than the roll out threshold field.

In FIG. 40, for example, to hold down (Hold Down Mode) a portion ofelectrode 30 as at 40--40 in FIG. 38, the three segment electrodes,i.e., Hold Electrodes 12(a), 12(c) and 12(e) are all at the samepotential E1', which is the same, or greater, than the potential E1,while the remaining two segments 12(b) and 12(d) (Roll Out Electrodes)are at an opposite potential E2=-E1', which is the same or greater thanthe potential E1, and moveable electrode 30 is at zero or groundpotential. The force in FIG. 40 is the same, or greater than the forcerequired to roll out the electrode 30. This keeps the electrode 30 fromrolling up. Because the potentials of neighboring electrodes areopposite; the fringing field effect will cause the electric field to bebelow the threshold field for rolling out wherever the membrane is notin contact with the fixed electrode, FIG. 41. Because of the fringingfield effect, changing the potential to the opposite polarity on threeof the fixed electrodes, i.e, the Hold electrodes, and leaving the othertwo at the same potential, the electric field between the Holdelectrodes and the moveable electrode is reduced in section 41--41 andnot reduced in section 40--40 compared with the case where all thepotentials are the same. The roll out forces are therefore reduced. Whenthe neighboring electrodes are of opposite potential, one can increasethe voltage on all five fixed electrodes and the moveable electrode willnot roll up, however one cannot increase the voltage too much, otherwisethe field in section 41--41 will become too high and the membrane willroll out. There will be some optimum voltage on the five electrodeswhich will lock the electrode. The locking effect will tend to bestronger if there are more electrodes which are narrower and moreclosely spaced to enhance the fringing effect. For example, subdividingthe fixed electrodes into seven electrodes with three hold electrodesand four roll out electrodes would probably give a strong hold effect.

Note that the preceding principles would apply equally to a structure inwhich the moveable electrode were segmented instead of the fixedelectrode.

A second method for providing a shutter which will hold in a partiallyrolled out position is to arrange for the contacting surfaces of themoveable electrode and the fixed electrode (or the insulation on thefixed electrode) to have a specified level of adhesion. Any time twomaterials come into very close proximity, there are usually attractiveforces (Van der Walls Forces).

The amount of force depends on the material, and surface structure. Withproper selection of the contacting materials, the adhesion between themoveable and fixed electrodes can be controlled to be low enough toallow the electrode to roll up, but high enough to significantly lowerthe roll up threshold voltage V_(RUT).

For a microshutter built in this way, it is possible to raise thevoltage applied to the roll out threshold V_(ROT), for a short period toroll the shutter part way out, and then drop the voltage to a lowerlevel (but not to zero) where the adhesion forces are holding theshutter partially rolled out. With the roll in this position, it ispossible to vary the voltage between the moveable and fixed electrode,between V_(RUT) and V_(ROT), without significant movement of theshutter.

With careful design and control of adhesion, this mode of operation willprovide grey scale in an X-Y scanned array for TV applications.

The optional PTFE, or some other material mentioned in the precedingembodiments, can serve as the adhesion control layer.

Floating Electrode Device

There is another version of the shutter light valve where the moveableelectrode 30 does not require an electrical contact. The moveableelectrode is electrically isolated, or floating, and the charge requiredin order to achieve the force to roll it out is produced by inducing thecharge with two or more fixed electrodes. One such electrode arrangementis shown in FIG. 43 (in the open position) and FIG. 44 (in the closedposition), where no electrical contact is made to the moveable electrode30, but there is a first fixed electrode 12(a) under the fixed part ofthe moveable electrode 30 and a second fixed electrode 12(b) right nextto the first electrode with a small gap G between. A voltage between thetwo fixed electrodes will roll out the electrode 30. In order tominimize the voltage required to roll out the electrode the capacitancebetween the first electrode and the membrane should be maximized bymaking the overlap area of the two electrodes large.

A second way of forming a floating electrode device is to split thefixed electrode in half forming two adjacent electrodes 12(c) and 12(d)beneath an insulator 14 (shown in partial section), as shown in FIG. 45.Applying a proper voltage V1 between these two electrodes 12(c) and12(d) will roll out the electrode 30.

Printer

Linear or two dimensional shutter arrays can be used for a class ofapplications which are generally referred to as spacial lightmodulators. Such applications include optical correlation, spectrumanalysis, crossbar switching, projection displays, printing, and neuralnetworks. One such example is the electrophotographic printer, whichwill now be described in detail in connection with FIGS. 46-48. Thisprinter 402 uses the same basic technology as a laser printer, exceptthat in this case, the print head 404 is formed of a light source 406and a linear microshutter array 408, in place of the scanned laser. Theprinter 402 is simpler and cheaper to build due to the elimination ofthe laser, and scanning lenses. The microshutter print head 404 consistsof a linear array 408 of shutters 430, each shutter 430 having a size ofaround 80 μm square. This small size provides for a resolution of 300dots per inch, which is excellent for printing text and sufficient forprinting photographs. The switching frequency of the microshutter 430can be well above 10,000 Hz, therefore, the printing speed will reachabove 30 inches per second at the 300 dots per inch resolution. Thehigher resolution required to achieve good photographic quality will bepossible using smaller shutters, and the print speed will remain aboutthe same because the smaller shutters will have a higher ultimatefrequency capability. Preferably, the light source 406 may be a vacuumfluorescent device directly above and in close proximity to themicroshutters 430. The microshutter array 408 is formed on a glasssubstrate which is thin, 0.003" or less, to allow the array to be placedvery close to a photoreceptor belt 410 and minimize the need to havecollimated light. Wire bond pads 416 are provided to address or makeelectrical connection to each pixel on shutter 430 with the digital datato be imaged from source 407. This data is input from source 407, oneline at a time.

The linear array 408 is used to print a pixel image line by line ontolight sensitive photoreceptive belt 410 as it is rotated by the printhead 404. The photoreceptor belt 410 is charged by corona from coronaunit 452. A pixel from one of the shutters 30 selectively discharges thecorona at a spot opposite the shutter. The resulting charge image isdeveloped with toner, from developer 440, that clings to charged spots.The toner image is transferred from the belt 410 to the paper 434 at thetransfer corona station 436 and fused to the paper at the fuser rollers438 to form a printed image. The image is removed from the belt 410 bycleaner 453.

DC Switch

The principles of the present invention can be used to form switches ofvarious kinds. One such switch is a D.C. switch 502 as shown in FIGS.49-53. This switch has special electrodes 504(a) and (b) i.e. currentcarrying electrodes which allow for metal to metal, or conductor toconductor contact. The conductor to conductor contact provides for alarge impedance change from an open to closed position compared with acapacitive type of switch (see microwave switch, to be described in nextsection), and in fact, the impedance change can be considerably largerthan a transistor. Because of this performance advantage and becausethese switches are at least as easy to fabricate as a transistor, theyhave the potential to replace the transistor for many applications.

A first embodiment of a DC switch 502 is shown in FIG. 49(a) in the openposition and FIG. 49(b) in the closed position. The switch is formed ona substrate 10 such as ceramic or glass or may be formed of a conductorwith an insulating layer 14 deposited thereon. A metal film such asaluminum is then coated on the insulator and patterned to form pull downelectrode 12(a) and a contact metallization area (not shown) where aspecial lower contact 504(a) of iridium or the like will be formed. Arelease layer is then formed such as by evaporation onto the structureand patterned to provide attachment areas A where the moveable electrode300 is to be formed. An upper contact 504(b) is formed on the releaselayer opposite the lower contact 504(a). Next, the moveable electrode isformed as previously described on the release layer and the releaselayer dissolved or dry etched away.

Electrical leads L1, L2 and L3 are attached to the pull down electrode16(a), lower contact 504(b) and moving member 300 respectively.Application of a DC voltage (e.g. 10 Volts as shown) across L1 and L3will cause moveable electrode to close as in FIG. 49(b) to connect lineL2 and L3 through contacts 504(a) and 504(b).

The concern with this type of switch, which is the same as for anymechanical relay, is the electrical arcing which can change or damagethe contacts, or even cause them to weld together. To avoid arcing onecould limit the application of this device to special applications wherethe switch is closed or opened only when there is no voltage across thecontacts. Once closed, current could pass without damaging the contacts.Alternatively one could use mercury wetting which is a common design forcontrolling the arc damage on contacts.

Another approach to solving the contact breakdown problem will bedescribed. But first, it is necessary to describe in some detail theelectrical breakdown effects when two contacts are either open orclosed. Consider the case of two contacts having a voltage applied tothem, as they begin to move toward each other. If the voltage is 100volts or less, a significant current can begin to flow whenever theminimum spacing between the two contacts is less than about 1micrometer. At this spacing the electric field has reached levelsgreater than one million volts per centimeter. Whether or not a currentflows depends greatly on the condition of the contacts. Because thecontacts are generally not perfectly smooth, the electric field canreach much higher levels at small protrusions. The high electric fieldcauses field emission of electrons from the electrically negativecontact, which then are accelerated and reach an energy of 100 electronvolts when they collide with the positive electrical contact. Theelectron flow will increase rapidly as the contacts move closer due tothe stored energy in the neighborhood of contacts, and the currentdensity can reach levels which are high enough to rapidly heat and meltthe contact metal. The damage is caused by electrons being acceleratedacross the very narrow gap.

In the second case where damage can occur, the contacts start out incontact with a current flowing between them, and the voltage between thecontacts is very low as they begin to separate. When the contactseparation reaches 10 or 20 Å the resistance begins to reach highlevels, and because of the natural inductance which occurs in anycircuit, the voltage across the contacts will rise rapidly. Depending onthe circuit the voltage can reach levels above the supply level. As withthe first case when the voltage gets high enough field emission willoccur, and melting of the positive electrode will also happen if thefield between the contacts gets high enough long enough.

To avoid this problem, the present switch 502 as shown in FIGS. 50(o),51, 52 and 53 is made to be very fast so the amount of time the contactsspend between contact and 1 micrometer apart, is very short. If thattime is short enough there is no longer time for enough electrons to beemitted and accelerated to melt the positive contact. Because theseswitches can be made very small and from very light materials, they arevery fast. The resonant frequencies of some of the smallest switchstructures reaches ten megahertz, which means that switch times of 100nanoseconds or less are possible.

Also to minimize contact damage the switch should move from a onemicrometer gap to a zero gap and back as quickly as possible. If onewere to place two switches in series and actuate them simultaneously,the total gap would change from two micrometers to zero and back in thesame time that one switch moved one micrometer. For the two switches inseries the gap changes about twice as fast. In a multi cell device wheremany switches are in series, the contact opening rate increases as thenumbers of switches. A highly reliable switch could have 100 switchcontacts, or elements, in series, each one have an opening rate of 1micrometer in 0.1 micro second. The total opening rate would be 1micrometer in 1 nanosecond which would reduce the energy dissipated to alow enough level to eliminate damage. The switching elements of thepresent invention are very small, and therefore many can be made on awafer. Also many of these 100 element series groups can be coupled inparallel to increase current capacity. If the one element consumes arearea of 5×5 micrometer, then a 10,000 element device can be placed on a1 millimeter square chip. Such a chip, depending on the design and thenumber of parallel and series combinations, would have the capacity toswitch 10 mA at 100 volts or 100 mA at 10 volts. A 4×4 array 520connected so that four parallel strings 410 of each switch element 502is shown in FIG. 54.

The fabrication details of an alternate single switch embodiment 502 ofthe array 520 are shown in FIGS. 50-53. Beginning with an insulatingsubstrate 10 or with a conducting substrate covered with an insulatinglayer, a metal film, such as aluminum, is formed and patterned usingthin film deposition and photolithography as described before to formboth the pull down electrodes 12(a) and 12(b) and the current carryingelectrodes 512 FIG. 50(a) and FIG. 50(b)! Contact metal 504, such asiridium, is applied to a contact area using thin film deposition andphotolithography as described before. The sample is covered with arelease layer 18, such as silicon, using evaporation as describedearlier, as shown in FIG. 50(c). The release layer is patterned toprovide attachment areas for the movable member 300 of the switch 502,as shown in FIG. 50(d). Coat and pattern an insulator layer 20 such asaluminum oxide which will be the bottom insulator layer of the movingmember, FIG. 50(e). Also coat and pattern with indium to provide anupper contact FIG. 50(f)!. Coat and pattern an aluminum layer 22, orsome other metal to provide the current conducting electrodes 530 andthe pull down electrodes 532 on the moving member FIG. 50(g), (h) and(i). Coat with final insulator layer 24 shown in FIG. 50(l). Pattern themoving member as shown in FIG. 50(m), to define the moving memberoutline. Place the sample in fluorine gas to remove the silicon torelease the moving member, FIG. 50(n). As shown in the embodiment ofFIGS. 50(o) and 51-53, the two pull down electrodes 12(a), 12(b), on thesubstrate 10 straddle the two current carrying electrodes 530 and 512.The pull down electrode 532, bridges over the current carrying electrode530, and is formed over the pull down electrodes 12(a), 12(b). Theinsulated layer 536 on electrode 532 is made from aluminum oxide. Thecurrent carrying electrodes are made from aluminum and the contactsthemselves are made from iridium. This type of design minimizes thecapacitance between the current carrying electrodes and the pull downelectrodes, and also minimizes the capacitance between the currentcontacts themselves when the are open. This switch device therefore hasgood isolation between the pull down circuit addressed by lines L1-L2and the current carrying circuit lines L3-L5, which is necessary forusing them in series.

Microwave Switch

With some small modifications the microshutter electrodes previouslydescribed can be used to make a microwave or high frequency switch. Thisswitch would form a variable capacitor in which the capacitance variesabout three orders of magnitude from open to closed. A low capacitanceresults in a high electrical switch impedance, and large capacitanceresults in a low impedance. The magnitude of the capacitances depends onhow many of the small rolling shutters are connected together inparallel. In order to operate as a switch at 10 Ghz, a capacitance rangefrom 0.02 to 20 pf from roll up to roll out would be a nominal designvalue. Lower or higher frequencies would require proportionally higheror lower capacitances to form effective switches.

A schematic drawing of a single element switch is given in FIGS. 54 and55, showing a fixed electrode 12 on substrate 10 and a moveableelectrode 30 on the insulating substrate in both the open and closedpositions. For convenience at high frequencies the backside of thesubstrate is also a conductor or electrode 31, which is grounded, sothat the first two electrodes 30 and 12 can serve as microstriptransmission line. The signal Vs used to operate the switches is a muchlower frequency, i.e., DC voltage, than the switched signal HF so thatthe operate signal Vs can be introduced directly across the coilelectrode 30 and fixed electrode 12 through a high frequency blockingfilter (inductor L1). A DC low frequency blocking filter capacitor C1 isalso provided in the high frequency (microstrip) line to block the lowfrequency switch signal Vs from traveling down the microstriptransmission line when the electrode is uncoiled when Vs goes from 0volts to +20 volts as shown in FIG. 55. The signal Vs used to operatethe switch is therefore very similar to the signal used to operate themicroshutter in previous embodiments.

FIGS. 56-59 show a multi element switch Sm designed to operate at 1.0Ghz. The capacitance change is from 0.03 to 300 pf and changes theimpedance from 0.5 to 500 ohms. The switching signal is fed in on thesmall lines L1 coming from the sides. The small lines L1 from theblocking inductors and the capacitors are outside the drawing and arenot shown. The moveable electrodes 30 are specially laid out to cause aminimum amount of change to the microstrip shape when the switch isclosed FIGS. 56, 57, which will minimize any reflections. When theswitch is open FIGS. 58, 59, the gap G will present a large impedance tothe signal traveling down the line, and will therefore reflect thesignal.

Equivalents

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

Note that aluminum has been indicated as a preferred material for use asthe moveable electrode because it has a low stress property. Many othermaterials can be substituted therefore if formed with sufficiently lowstress. The main priority is to form the moveable electrode with thestress built in as formed. The stress biases the electrodes to rotate inthe proper direction. The corrugations provide stiffness in a givendirection to prevent the moveable electrode from curling or twisting asit coils. An alternative to the corrugations is to provide anisotropicstress in the electrode films or to use stress strips as in FIGS. 20-23or FIG. 37.

In the preferred embodiment, the moveable electrode is formed of threelayers with the stress changing across the electrode from tensile in thetop aluminum oxide layer to about zero stress in the middle aluminumlayer and compressive in the lower aluminum oxide layer. All of this isaccomplished as the layers are deposited. The average stress is fairlylow.

What is claimed is:
 1. A device comprising:a) a fixed electrode; b) afirst high resistance layer on said fixed electrode; and c) a moveableelectrode formed with a stress and affixed at one end and rotationallymoveable about a single axis toward said fixed electrode by applicationof a suitable voltage to the device and rotatable in an oppositedirection upon removal of said voltage, said stress biasing saidmoveable electrode toward movement in said opposite direction, saidmoveable electrode having a second high resistance layer formed betweensaid moveable electrode and said first high resistance layer.
 2. Thedevice of claim 1 wherein the moveable electrode is stiffened to avoidcurling transverse the direction of rotation thereof.
 3. The device ofclaim 1 wherein the moveable electrode rotates into a roll and isanisotropically stressed in the direction of roll.
 4. The device ofclaim 1 wherein the moveable electrode is anisotropically stiffened in adirection transverse the direction of rotation.
 5. The device of claim 1wherein the moveable electrode is formed of a film having compressivestress on a side facing the fixed electrode and tensile stress on anopposite side of the film and wherein the net stress of the moveableelectrode is low.
 6. The device of claim 5 wherein the moveableelectrode is formed of outer layers of insulator material and an innerconductive layer.
 7. The device of claim 6 wherein the moveableelectrode has a thickness of less than about 3000 Angstroms.
 8. Thedevice of claim 1 in which the moveable electrode modulates light. 9.The device of claim 1 wherein the moveable electrode switcheselectricity.
 10. The device of claim 1 wherein the moveable electrodeforms a light shutter.
 11. A device comprising:a) a substrate; b) afixed electrode on the substrate; c) a first high resistance bodycovering a surface of the fixed electrode opposite the substrate; and d)a moveable electrode fixed at one end and stressed, as formed, to rollup in a coil, said moveable electrode having a second high resistancebody formed between said moveable electrode and said first highresistance body; and wherein upon application of an electric potentialto the device, the moveable electrode unrolls in a preferential rolldirection to cover a substantial portion of the fixed electrode.
 12. Thedevice of claim 11 wherein the fixed electrode is comprised of a thintransparent layer of material from the group comprising indium-tinoxide, gold, copper, silver, platinum or tungsten and the moveableelectrode, substrate, and high resistance body are transparent and anon-transparent layer is formed over the moveable electrode and thesecond high resistance body is formed on the moveable electrode on aside of the moveable electrode closest to the fixed electrode.
 13. Thedevice of claim 12 wherein the first and second high resistance bodiesare formed of material from the group comprising aluminum oxide, silicondioxide or silicon nitride.
 14. The device of claim 11 wherein themoveable electrode is stiffened in a direction transverse the rolldirection to cause the moveable electrode to unroll in the preferentialroll direction.
 15. The device of claim 14 wherein the stiffening isachieved by providing corrugations which extend transverse the rolldirection.
 16. The device of claim 11 wherein the moveable electrode isformed of three layers comprising a middle layer of metal and outerlayers of insulator.
 17. The device of claim 16 wherein the net stressof the three layers is low.
 18. The device of claim 11 wherein theelectrodes switch electricity.
 19. The device of claim 16 wherein thetwo insulator layers are formed of aluminum oxide and the metal layer isformed of aluminum.
 20. The device of claim 11 wherein the moveableelectrode modulates light.
 21. A light modulator comprising:a) a fixedelectrode; b) an insulator over the fixed electrode; and c) a moveableelectrode adjacent said fixed electrode and attached at one end to saidinsulator which moveable electrode is rotatable about said one end andbiased, as formed, in a manner which causes said moveable electrode tocoil in a direction away from said fixed electrode, wherein a firstresistive layer is formed on a surface of the fixed electrode facing themoveable electrode and a second resistive layer is formed on a surfaceof said moveable electrode facing the fixed electrode.
 22. The modulatorof claim 21 wherein the bias is caused by anisotropic stress created informing the moveable electrode.
 23. The modulator of claim 21 whereinthe bias is caused by anisotropic stiffening of the moveable electrode.24. The modulator of claim 21 wherein the bias is caused by anisotropicstiffening provided by corrugations extending transverse the directionof coiling movement.
 25. The modulator of claim 21 wherein thecorrugations are in the form of non-uniform ridge spacings.
 26. A lightmodulator comprising:a) a fixed electrode; and b) a moveable electrodeadjacent said fixed electrode which moveable electrode isanisotropically stiffened, as formed, in a manner which tends to causesaid moveable electrode to coil in a preferred coil direction.
 27. Themodulator of claim 26 wherein the movable electrode is stiffened byforming corrugations which extend transverse the preferred rolldirection.
 28. A light valve comprising:a) a substrate; b) a fixedelectrode on the substrate; and c) a moveable electrode fixed at one endand anisotropically biased to roll up in a coil in a preferentialdirection; and wherein upon application of an electric potential themoveable electrode unrolls in an opposite direction to cover asubstantial portion of the fixed electrode.
 29. The valve of claim 28wherein the fixed electrode is comprised of a thin layer of materialfrom the group comprising indium tin oxide, gold, silver, platinum ortungsten.
 30. The valve of claim 28 wherein the moveable electrode isbiased by being stiffened in a direction transverse the roll directionto prevent the moveable electrode from curling transverse the rolldirection.
 31. The valve of claim 28 wherein the stiffening is achievedby providing corrugations which extend transverse the roll direction.32. The valve of claim 28 wherein the moveable electrode is formed of amiddle conductive layer with insulative layers on either side thereof.33. The valve of claim 32 wherein the net stress of the movableelectrode is low.
 34. A light valve comprising:a) a substrate; b) afixed electrode on the substrate; c) a moveable electrode fixed at oneend and anisotropically biased to roll up in a coil in a preferentialdirection; and wherein one of said electrodes is divided into spacedapart conductive strips and wherein upon application of a first electricpotential across said fixed and moveable electrodes the moveableelectrode unrolls in an opposite direction to cover a portion of thefixed electrode; and upon application of a second electric potentialacross an electrode and one or more conductive strips, the moveableelectrode assumes a fixed position.
 35. A spatial light modulatorcomprising:a) an array of light valves, each valve comprising a fixedelectrode and a moveable electrode and wherein the moveable electrode isfixed at one end and anisotropically biased to roll up in a coil in apreferential direction; and wherein upon application of an electricpotential across said electrodes, the moveable electrode unrolls in anopposite direction to cover a substantial portion of the fixedelectrode; b) an array of address pads coupled to a respective one ofsaid valves to apply said potential across said electrodes; and c) asource of light adjacent said array for projecting light at said arrayto form a pixel image in space, each pixel thereof formed in response tothe respective potentials applied to said electrodes.
 36. A microwaveswitch comprising:a) a fixed conductive member and a moveable conductivemember with an insulator formed therebetween and wherein the moveablemember is fixed at one end and is anisotropically biased to roll up in acoil in a preferential direction; and wherein upon application of anelectric potential across said members, the moveable member unrolls inan opposite direction to contact a substantial portion of the fixedelectrode; b) an input lead coupling a source of microwave energy to onemember; and c) an output lead coupled to the remaining member;wherebywhen the moveable member unrolls, a large capacitance is formed acrossthe switch, producing low impedance coupling of the microwave energyfrom the source to the output lead.
 37. A DC switch for switching DCvoltage comprising:a) an array of switches, each switch comprising afixed current carrying electrode on a substrate and a moveablecantilevered electrode fixed at one end with opposed contact areas onsaid moveable electrode and said fixed electrode; b) at least onepotential supplying electrode; andwherein upon application of anelectric potential to said potential supplying electrode, the moveableelectrode moves in a direction to bring the contact areas in contact.38. The switch of claim 37 in which the potential supplying electrode isformed on the substrate and consists of two spaced apart conductivestrips.